1. Field of the Invention
The present invention relates generally to a method and apparatus for encrypting and compressing multimedia data, and more particularly to a method and apparatus for encrypting and compressing multimedia data, which transforms and compresses multimedia data through a certain encryption key in a process of compressing the multimedia data to record and transmit the multimedia data, and which encrypts the multimedia data to allow the multimedia data to be decoded using only the encryption key used in the compressing process. The present application is based on Korean Patent Application No. 2002-72813, which is incorporated herein by reference.
2. Description of the Related Art
Moving Picture Experts Group (MPEG) standards are international standards for methods of compressing and storing, and transmitting moving images and audio data, and, methods of decompressing, processing and coding compressed information. The MPEG standards include MPEG-1, MPEG-2 and MPEG-4. Of the MPEG standards, MPEG-1 (ISO/CEI 11172), which is the most basic method of compressing multimedia data, eliminates or transforms duplicated information and compresses moving image signals by applying statistical properties to the remaining or transformed multimedia data.
MPEG standards use Discrete Cosine Transform (hereinafter referred to as “DCT”) and quantization to remove spatial redundancy, use Differential Pulse Code Modulation (DPCM) to remove temporal redundancy, and additionally use entropy encoding including Run Length Coding (RLC) and Hoffmann coding.
Additionally, MPEG standards basically include Groups Of Pictures (GOPs) each of which is a set of frames. Each of the GOPs includes an Intra-frame (I-frame), a forward Predicted frame (P-frame) and a Bi-directional predicted frame (B-frame).
Recently, as a wireless communication technology has developed and a mobile communication technology has been widely used, multimedia services based on the compressing method have been offered, and therefore the security of data provided through the multimedia services has been further required.
That is, corresponding services should be provided only to users having service use rights. For example, movies are transmitted only to users having paid certain fees, or image information is transmitted only to users having rights by which they participate in secret conferences.
The Data Encryption Standard (hereinafter referred to as “DES”) adopted as an international standard algorithm in 1997, is generally used as an encryption algorithm.
DES is a block encryption algorithm that processes a block unit of plain text using a symmetric key, and is used to transmit and reproduce compressed multimedia data for providing secure multimedia services.
U.S. Pat. No. 6,021,199 entitled “Motion picture data encrypting method and computer system and motion picture data encoding/decoding apparatus to which encrypting method is applied” discloses a method of encrypting multimedia data using an MPEG compression method that selectively encrypts the I-frames of MPEG data through DES, using the property of the I-frame to include original image information, thus reducing the amount of data.
A process of encrypting multimedia data using such an MPEG compression method is carried out as shown in FIGS. 1 and 2.
That is, many of the values of an 8×8 block become 0 through DCT and quantization processes, as shown in FIG. 1.
To efficiently process the frame data described above, the values of Differential Coefficients (hereinafter referred to as “DC coefficients”) and Amplitude Coefficients (hereinafter referred to as “AC coefficients”) are read in a zig-zag order, for example, in the order of DC, AC1, AC2, . . . , and AC63, compressed through an entropy encoding process (100), and encrypted through the DES encryption process (200).
Additionally, as shown in FIG. 2, a multimedia data producer receives a public key sent from multimedia data receivers at step 1, and generates a symmetric key needed to decode encrypted multimedia data provided through multimedia services, encrypts the generated symmetric key using the public key sent from the receiver and sends the symmetric key to the receiver at step 2.
The producer periodically changes the symmetric key used in the DES and therefore improves the security of data at step 3.
A method of encrypting MPEG data using the shared symmetric key according to a DES algorithm requires resources for processing encrypting and decoding processes because the encrypting and decoding processes are complicated.
Additionally, since the method cannot improve a data compression ratio of multimedia data, the method is not suitable for real time multimedia services provided to wireless mobile terminals.
Accordingly, there have been demands for a multimedia security system that efficiently handles limits of the bandwidth resources of a wireless network environment and limits of the computation resources of a mobile terminal.